There are today many mobile communication systems that use synchronised time division duplex (TDD) schemes. One example of this is Time Division-Long Term Evolution (TD-LTE). These types of systems have certain message structures, such as frames or subframes provided after one another in time. The communications made in these systems, which involve base stations, each communicating in one or more cells, are usually synchronised with each other. The base stations in the different cells transmit signals in one period of time, often denoted downlink period, and receive signals in another period, often denoted uplink period. A TD-LTE frame contains 10 subframes with different arrangements of uplink and downlink subframes. A 2nd and a 7th subframe are denoted special subframes and include a downlink part DwPTS, a guard period GP and an uplink part UpPTS. The downlink and uplink parts (DwPTS and UpPTS) should be sufficiently separated from each other in order to avoid interference between uplink and downlink. If systems are synchronised this means that all base stations are transmitting at the same time and all are receiving at the same time.
One problem that has received increased attention lately in relation to systems using synchronised time division schemes are problems caused by atmospheric ducts.
An atmospheric duct is a horizontal layer that is created in the lower atmosphere, typically the troposphere. In such a duct the vertical refractive index gradients are such that radio signals (and light rays) are guided or ducted along the length of the duct. The radio signals in the ducts therefore tend to follow the curvature of the Earth. The radio signals also experience less attenuation in the ducts than they would if the ducts were not present.
The occurrences and locations of these ducts are hard to predict. They occur rarely, typically a few days or a number of hours in one year, and also often occur in some special area, like in the desert or near the sea.
For a mobile communication system, an atmospheric duct will cause downlink (DL) signals from far away base stations to travel through the atmosphere with long transmission delay but with a very low attenuation. This delayed but still strong signal may seriously affect TDD system performance.
Since the base stations in the above mentioned systems are supposed to transmit in well controlled periods of the frame structure and to receive signals from mobile stations in other periods of the frame structure, it is possible that the downlink signals from a base station travelling through an atmospheric duct will reach another base station when this other base station is supposed to receive signals from mobile stations, i.e. when this other base station is in uplink communication.
This situation can be serious because base stations transmit with much more power than a mobile station. Since the signals in the atmospheric duct have low attenuations, this means that a base station may not be able to receive any signals from mobile stations in the whole or parts of the uplink time assigned to reception of signals from mobile stations. This means that one of the fundamental base station functions may be impossible to perform.
It is also possible for a base station to interfere with uplink communication of another base station for other reasons, like if it has a faulty operation, like having lost its synchronisation or because a new base station has been added to the system.
There exist some prior art documents that discuss adjusting the frame structure based on interference.
US 2010/097964 describes how a central node controls the adapting of the duration of a guard period according to measured interference.
US 2010/097963 describes measuring of the interference level in a cell for part of the time between two consecutive down link periods and varying the duration of the first or second guard period, the duration of the up link period or the traffic sent in the up link period in accordance with the measured interference level.
WO 2008/115105 describes how base stations utilize time information from one of a number of synchronization sources in order to time the down link, up link and guard periods properly. A base station is here allowed to use information on the inaccuracy of a synchronization source of the base station in order to adapt its downlink periods so that it does not interfere with uplink periods in one or more other base stations.
WO 2008/103090 discloses measuring the interference level during part of the time between two consecutive down link periods and varying the duration of guard periods according to the interference level.
Most ways of reducing interference between base stations relies on a central node controlling the reduction or on information obtained from another source. It would however be of interest to obtain a way to reduce interference without having to involve a central node. The amount of communication can be extensive if a central node is employed. If many base stations experience interference, the communication network used may be slowed down. The central node may also take some time in determining the correct action. Furthermore the functionality may be lost in case the central node is defective, which will influence all base stations experiencing interference.
There is therefore a need for a solution to this problem.